An important part of many modern pollution control schemes for use with power plants, paper mills, etc., is a determination of the amount (mass) of exhaust flowing through the exhaust stack. To determine the mass flowing through the stack, it often is necessary to first determine the velocity of the fluid flowing in the stack. One traditional way of measuring fluid velocity in the stack is to use one or more pitot tubes positioned in the stack. Unfortunately, this arrangement generally suffers from requiring a large number of such tubes in order to approximate the average velocity of the fluid flowing through the stack.
In recent years, the so-called "optical convolution velocimeter" has been developed as an alternative to the use of pitot tubes. Early work in this area focused on the measurement of air speed over a moving aircraft. For example, U.S. Pat. No. 3,953,126 of Kim, et al relates to an optical convolution velocimeter having a flow channel and which can be attached to the exterior of an airplane for measuring the speed of the air flowing through the flow channel. The patent discloses that a light intensity pattern formed by light traversing the flow channel is subjected to spacial convolution by nonhomogeneities (such as non-uniform gas densities) as a result of the fluid flow. The convolution is disclosed as generating a new light intensity response function which is periodic and which has a base frequency which is directly proportional to the flow velocity. Parallel rays of light are passed through the flow channel perpendicular to the flow and subsequently through an artificially introduced transfer function device, such as a grating or a doubly-reflective mirror.
A research and development report, No. EPA-600/2-79-192, dated October, 1979, and entitled "Cross Stack Optical Convolution Velocimeter" by M. J. Rudd, published by the United States Environmental Protection Agency, discussed the general applicability of some of the principles disclosed in U.S. Pat. No. 3,953,126 to the measurement of a line average of a stack gas velocity as a means of gathering information for use in pollution control systems. However, the laboratory study discussed in the report generally fails to present a description of an apparatus which would be easily serviced in the field, reliable in operation, and relatively immune to electrical interference. This is due in part to the disadvantageous placement of the optical, mechanical, and electrical/electronic components on the stack according to the apparatus disclosed in the report. Such an arrangement has been found in practice to be unreliable and difficult to service, particularly since such stacks are often quite tall and are naturally exposed to inclement weather, including lightning strikes.
Furthermore, it has been typical to "standardize" the output of known velocimeter apparatuses used on stacks rather than performing a "dynamic calibration" in which the measured signal is compared with a signal representing a known velocity.
Accordingly, it can be seen that a need yet remains for a velocimeter apparatus for measuring fluid velocity in a stack which is easily serviced, reliable in operation under adverse conditions, capable of being precisely calibrated once installed, and which is relatively immune to interference effects. It is to the provision of such a velocimeter apparatus that the present invention is primarily directed.